my favortie upgrade is repel! I thought of two upgrades you might consider, I know you probably have you're hands full, but they might intrest you. one could be where the clouds all shoot giant lightning bolts that destroy all the zombies they touch! You could also make one where the zombies all slow way down for 1 second for some points, which you could buy in the pause menu.
Just an idea.

For the Blaster, maybe it could supercharge all Laser weapons, like the M16, Shining Lance, and Shining spreader?

By the Way, I added 2 pictures that could be used as some kind of title or cover. One has the Words Resident Penguin, One is For adding you're own text if you want to use them and you don't like the look of the words. hope you like them!